It is often desirable to measure temperature, pressure or volume changes of a target and numerous sensor technologies are available for making such measurements.
Thermistors are typically used to measure temperature. The resistance of the thermistor changes in response to changes in temperature and can thus be used to determine the corresponding temperature change. Another alternative is to use a thermocouple which produces a temperature-dependent voltage across a connection between two conductive substrates.
To measure pressure, many sensors utilise a diaphragm which is deformed by changes in pressure. The diaphragm deformation may then cause changes in the resistivity of attached strain gauges or in the output voltage of an attached LVDT. Another alternative is for the diaphragm to form part of a capacitor, changes in deformation in the diaphragm causing corresponding changes in capacitance. The changing signals are then used to calculate the corresponding change in pressure.
Ultrasonic and microwave sensors emit ultrasonic waves and microwaves respectively and may detect a return signal which is reflected from a reflective surface of a target. Differences between the emitted and reflected signal can then be used to determine the level of the surface and this can be used to determine the volume of the target.
Eddy current probes are suitable for detecting the level of a conductive surface. Eddy current probes produce a first electromagnetic field which in turn induces an alternating current in the conductive surface. The current produces a second electromagnetic field which interacts with the first electromagnetic field. The resultant electromagnetic field is detected by the probe and may be used to determine the distance between the probe and the electrical conductor.
However, many of these known sensors include complex electronics and/or silicon components which cannot be used in high temperature environments.